Chemical transformations of poplar lignin during co ... - ACS Publications

toluene/ethanol for 8 h. The extractives-free poplar was then ball-milled in a porcelain jar with. 159 ceramic balls via Retsch PM 100 (Newton, PA) at...
0 downloads 0 Views 893KB Size
Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Article

Chemical transformations of poplar lignin during cosolvent enhanced lignocellulosic fractionation process Xianzhi Meng, Aakash Parikh, Nikhil Nagane, Bhogeswararao Seemala, Rajeev Kumar, Charles M. Cai, Yunqiao Pu, Charles E. Wyman, and Arthur Jonas Ragauskas ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01028 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

1

Chemical Transformations of Poplar Lignin during Co-Solvent

2

Enhanced Lignocellulosic Fractionation Process

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Xianzhi Meng†, Aakash Parikh‡,§, Bhogeswararao Seemala‡,§, Rajeev Kumar‡, Yunqiao Pu⊥, Phillip Christopher§,#, Charles E Wyman‡,§, Charles M. Cai‡,§, Arthur J. Ragauskas*,†,⊥,∥ †

Department of Chemical & Biomolecular Engineering, University of Tennessee Knoxville, Knoxville, TN 37996, USA ‡ Bourns College of Engineering – Center of Environmental and Research Technology (CE-CERT), University of California, Riverside, CA 92507, USA § Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, Riverside, CA 92521, USA # Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106, USA ⊥ Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ∥Department of Forestry, Wildlife, and Fisheries; Center for Renewable Carbon, The University of Tennessee Knoxville, Institute of Agriculture, Knoxville, TN 37996, USA

Corresponding Author: * Arthur J. Ragauskas. Fax: 865-974-7076; Tel: 865-974-2042; E-mail: [email protected]

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

39

Page 2 of 26

ABSTRACT

40

Converting lignocellulosic biomass to ethanol is significantly hindered by the innate

41

recalcitrance of biomass, and it usually requires a pretreatment stage prior to enzymatic

42

hydrolysis. A promising novel pretreatment named Co-Solvent Enhanced Lignocellulosic

43

Fractionation (CELF) that involves moderate temperature reactions with a dilute acid in

44

THF-water mixture was recently developed to overcome biomass recalcitrance. Detailed

45

elucidation of physicochemical structures of the fractionated lignin that is precipitated after

46

CELF pretreatment, also called CELF lignin, reveal transformations in its molecular weights,

47

monolignol composition, and hydroxyl groups content. Isolated CELF lignin revealed dramatic

48

reductions in its molecular weight by up to ~90% compared to untreated native lignin.

49

Furthermore, CELF lignin’s β-O-4 interunit linkages were extensively cleaved after CELF

50

pretreatment as indicated by a semi-quantitative HSQC NMR analysis. This is further evidenced

51

by a 31P NMR analysis showing a significant decrease in aliphatic OH groups due to the

52

oxidation of lignin side chains, whereas the content of total phenolic OH groups in CELF lignin

53

significantly increased due to drastic cleavage of interunit linkages. In conclusion, CELF process

54

generated a uniquely transformed pure lignin feedstock of low content aryl ether linkages, low

55

molecular weight, and high amount of phenolic hydroxyl groups, suitable for its development

56

into fuels, chemicals, and materials.

57

KEYWORDS

58

Biomass recalcitrance, Co-Solvent Enhanced Lignocellulosic Fractionation, Antioxidant, Lignin

59

Valorization 2

ACS Paragon Plus Environment

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60 61

ACS Sustainable Chemistry & Engineering

INTRODUCTION Lignocellulosic biomass represents a promising sustainable platform to fossil resources.1 Long

62

term driving forces including environment concerns and energy security have promoted

63

worldwide biofuel development.2 Nevertheless, there are still significant challenges associated

64

with biofuel commercialization. Biomass recalcitrance represents one of the critical challenges

65

for biological biorefining of biomass, therefore it is vital to develop cost-effective pretreatment

66

techniques to enhance sugar release performance through enzymatic hydrolysis. Over the past

67

decade, various pretreatment technologies have been developed to overcome biomass

68

recalcitrance and each pretreatment has its own target, but all have been proven to change the

69

plant cell wall structure.3, 4 Lignin, a three dimensional cross-linked polyphenolic polymer, has

70

been considered as the most recalcitrant component of the three primary biopolymers present in

71

plant cell walls for the conversion of biomass to biofuels.5 With its rich polyaromatic structure,

72

lignin has significant potentials to serve as a sustainable feedstock for the production of

73

renewable chemicals and fuels.6 Thus, many pretreatments have been developed and modified

74

with an effort of removing or at least redistributing lignin across the plant cell wall.7

75

Organosolv pretreatment is a process that usually involves addition of organic solvents such as

76

ethanol, methanol, or acetone as modifiers or co-solvents to the biomass under specific

77

conditions of pressure and temperature in the presence of an acid.7 Recently, tetrahydrofuran

78

(THF) has been identified as a new multifunctional solvent for enhancing the pretreatment of

79

biomass to obtain higher yields of sugars and other fuel precursors as well as improving the

80

overall utilization of biomass for conversion to fuels. Traditionally, THF was used to dissolve 3

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

81

acetylated lignin for molecular weight analysis in biomass research.8 Notably, THF can be

82

considered as a renewable solvent as it can be manufactured from maleic anhydride,

83

1,4-butanediol or furfural that are catalytically produced from C5 sugars.9-11 THF is also

84

inherently biodegradable, thus it is not expected to be environmentally persistent.12 Co-Solvent

85

Enhanced Lignocellulose Fractionation (CELF) pretreatment technology was developed as a new

86

generation pretreatment that involves reacting biomass with dilute acid in mixtures of

87

THF-H2O.13-15 At reaction temperatures of 170 oC or higher, CELF pretreatment could produce

88

high yields of fuel precursors including furfural (FF), 5-hydroxymethylfurfural (5-HMF), and

89

levulinic acid (LA) directly from biomass.13 For example, employing dilute FeCl3 in CELF

90

reaction of hardwood chips resulted in co-production yields of >90% FF and >50% 5-HMF

91

simultaneously.16 At milder temperatures below 160 oC, CELF can be used to achieve nearly

92

complete recovery (>95%) of fermentable C5 and C6 sugars from biomass if the pretreated

93

solids were subsequently treated using enzymes at extremely low dosages.14 Combining CELF

94

pretreatment with simultaneous saccharification and fermentation (SSF) dramatically improved

95

ethanol fermentation yields at high biomass solids loadings.17 In all cases, between 85-95% of

96

the acid insoluble “Klason” lignin can be removed from biomass during CELF pretreatment,

97

resulting in the precipitation of a very clean lignin product, also called CELF lignin, from the

98

liquid phase after recovery of the THF by low-temperature vacuum distillation.13, 14, 16

99

With all of the potential applications of lignin as renewable sources for production of fuels,

100

materials, and chemicals, understanding its fundamental characteristics is of significant

101

importance.6 During dilute acid or aqueous hydrothermal pretreatments, lignin deposits typically 4

ACS Paragon Plus Environment

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

102

onto cellulose surface as droplets.18-20 On the other hand, it has been reported that lignin adopts

103

extended coil configurations during CELF process and is preferentially solvated by THF as

104

determined by all-atom molecular-dynamics (MD) simulation.21 A further replica exchange MD

105

simulation study by Smith et al. showed that lignin was a coil regardless of being in the miscible

106

or immiscible temperature region, suggesting CLEF pretreatment may be performed at lower

107

temperatures and pressures.22 It was reported that CELF pretreatment increased S lignin fraction

108

and reduced G lignin in solid residue for poplar and corn stover, and in fact, CELF pretreatment

109

almost completely removed corn stover G units.23 However, the detailed structural

110

characterization of CELF lignin which is the lignin dissolved in THF, accounting for over 80%

111

of lignin in raw material, has not been reported prior to this paper to the best of our knowledge.

112

In this study, the structure of poplar lignin extracted from CELF pretreatment at different

113

conditions was determined following well-established methodologies including GPC and NMR,

114

and the results were compared to the native (untreated) lignin to determine the exact effect of

115

CELF process on the chemical structure of lignin.

116

MATERIALS AND METHODS

117

Feedstocks and Chemicals. Poplar (Populus trichocarpa x deltoids) was provided by

118

National Renewable Energy Laboratory (NREL, Golden, CO) and size reduced in a laboratory

119

mill (Model 4, Arthur H. Thomas Company, Philadelphia, PA) to obtain less than a 1 mm

120

particle size. The composition of untreated poplar was measured to be ~45% glucan, ~14% xylan,

121

and ~22% Klason lignin using NREL laboratory analytical procedure. Ferric chloride was

122

purchased from Sigma Aldrich (St. Louis, MO, US) and reagent grade THF was purchased from 5

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

123

Fisher Scientific (NJ, US). All the chemicals used in this study were used as received without

124

any further purification.

125

Page 6 of 26

Co-solvent Enhanced Lignocellulosic Fractionation Pretreatment. CELF pretreatments at

126

different severities were performed in a 1 L Parr reactor (236HC Series, Parr instruments

127

Company, Moline, IL) equipped with 6-blade impellers operating at 200 rpm driven by an

128

electric motor. The Parr reactor was heated using a 4 kW fluidized sand batch (Model SBL-2D,

129

Techne Princeton, NJ, US), and the temperature was measured by an in-line thermocouple

130

(Omega, K-type).13 The co-solvent mixture was prepared by adding THF to water in increasing

131

volumes starting from 1:1 (THF 50% v/v) to 7:1 (THF 87.5% v/v) THF-to-water ratios. Biomass

132

and acid catalyst (FeCl3 or H2SO4) loadings based on the total mass of the reaction mixture and

133

anhydrous mass were 5 wt% and 1 wt%, respectively. After pretreatment, the reactor was

134

quenched in a water bath at room temperature. The hydrolysate was then separated from the

135

solids using vacuum filtration through a glass fiber filter paper (Fischer Scientific, Pittsburg,

136

PA). The CELF pretreatment conditions applied in this work are shown in Table 1.

137 138 139 140 141 142 143 6

ACS Paragon Plus Environment

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

144

ACS Sustainable Chemistry & Engineering

Table 1. List of pretreatment conditions applied in this work for CELF process of poplar. Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Temperature (oC) 160 180 170 170 180 180 180 180 180 180 180 180 180 180 180

Time (min) 15 20 30 60 20 20 20 20 20 20 20 20 20 15 30

Catalyst loading (M) 0.1 H2SO4 0.1 H2SO4 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.05 FeCl3 0.15 FeCl3 0.1 FeCl3 0.1 FeCl3

THF: water (v:v) 1:1 4:1 3:1 3:1 1:1 2:1 3:1 4:1 5:1 6:1 7:1 4:1 4:1 3:1 3:1

145 146

CELF lignin isolation. CELF lignin was isolated from the liquid product by room

147

temperature vacuum distillation according to literature procedures (Scheme 1).16 Once the THF

148

was removed from the aqueous phase, the precipitated sticky dark resinous solid lignin was

149

rinsed with water and diethyl ether to remove non-lignin soluble impurities. The purified lignin

150

was then dried at 40 oC for 15 h and the dry weight was measured by a moisture content

151

analyzer. The composition of isolated lignin was determined according to a NREL analytical

152

procedure.24 All the CELF lignin samples were almost carbohydrate free, and presented a high

153

total lignin content above 90%. The detailed composition analysis of CELF lignin are presented

154

in Table S1 in supporting materials.

7

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomass THF:Water Acid Catalyst

CELF pretreatment

Solid-liquid Separation

Cellulose rich solids

Vacuum distillation

Liquid hydrolysate

155 156 157

Page 8 of 26

Water

Precipitated sticky dark resinous lignin

Filtration Filtration and diethyl ether wash

Dried at 40 oC for 15 h

Solid lignin

Dry lignin powder

Scheme 1. Schematic diagram of CELF lignin isolation. Cellulolytic Enzyme Lignin Isolation. Cellulolytic enzyme lignin (CEL) was isolated from

158

poplar according to a modified method.25 In brief, poplar was Soxhlet-extracted with

159

toluene/ethanol for 8 h. The extractives-free poplar was then ball-milled in a porcelain jar with

160

ceramic balls via Retsch PM 100 (Newton, PA) at 600 rpm for 2.5 h followed by enzymatic

161

hydrolysis in acetate buffer (pH 4.8, 50 °C) using Cellic CTec 2 cellulase and HTec 2

162

hemicellulase (1:1, mass ratio), as the enzymes (150 mg protein loading/g biomass) for 48 h. The

163

solid residue was isolated by centrifugation and hydrolyzed again with freshly added buffer and

164

enzymes at same loadings. To remove any remaining cellulases, the lignin-rich residue was then

165

treated with Streptomyces griseus protease (Sigma-Aldrich) at 37 °C overnight followed by a 10

166

min deactivation at 100 °C. After filtration, the solid residue was extracted twice with 96% (v/v)

167

p-dioxane/water mixture at room temperature for 48 h. The extracts were combined, rotary

168

evaporated, and freeze-dried to recover CEL.

169

Lignin Molecular Weight Distribution Analysis. The molecular weight analysis was

170

performed as previously described.26 Oven-dried lignin samples (~5 mg) were acetylated with a

171

2.0 mL acetic anhydride/pyridine (1:1, v/v) mixture at room temperature. After 24 h, ethanol was

172

added to the reaction and left for 30 min. The solvent was then subjected to rotary evaporation 8

ACS Paragon Plus Environment

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

173

under reduced pressure. The whole process was repeated twice until all the acetic acid was

174

removed from the solution. The derivatized lignin was then dried at 45 °C overnight in a VWR

175

1400E vacuum oven and then dissolved in THF (1 mg/mL) overnight prior to the GPC analysis.

176

The molecular weight analysis was analyzed by an Agilent GPC SECurity 1200 system equipped

177

with four Waters Styragel columns (HR1, HR2, HR4, and HR6) and a UV detector (270 nm).

178

Polystyrene narrow standards were used to prepare the calibration curve, and THF was used as

179

the mobile phase at a flow rate of 1.0 mL/min. The injection volume was set at 30 µL. The

180

number-average and weight-average molecular weights were calculated using a WinGPC Unity

181

software. While polystyrenes are widely used as standards to generate the relative calibration

182

curve in GPC analysis, high molecular weight (MW) polystyrene standard and high MW lignin

183

are not necessary eluted at the same elution volume, and the molecular weight should be taken

184

with a measure of caution.

185

Lignin NMR Characterization. Two-dimensional 13C-1H HSQC and 31P NMR experiments

186

were both acquired with a Bruker Avance 400 MHz spectrometer according to a previously

187

published literature.26 A standard Bruker pulse sequence was used under the following

188

conditions: 210 ppm spectral width in F1 (13C) dimension with 256 data points and 11 ppm

189

spectral width in F2 (1H) dimension with 1024 data points, a 90° pulse, a 1JC-H of 145 Hz, a 1.5 s

190

pulse delay, and 32 scans. ~50 mg of dry lignin samples were dissolved in deuterated DMSO for

191

HSQC experiments. Quantitative 31P NMR spectra were acquired using an inverse-gated

192

decoupling pulse sequence (Waltz-16), 90° pulse, 25 s pulse delay with 128 scans. ~20.0 mg of

193

lignin samples were dissolved in a solvent mixture of pyridine and deuterated chloroform 9

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

194

(1.6/1.0, v/v, 0.50 mL). The lignin solution was then further derivatized with 0.075 mL

195

2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP). Chromium acetylacetonate and

196

endo-N-hydroxy-5-norbornene-2,3-dicarboximide (NHND) were also added to the solution as

197

relaxation agent and internal standard, respectively. All the data was processed using the

198

TopSpin 2.1 software (Bruker BioSpin).

199

Page 10 of 26

Error Analysis. Error bars shown in each figure represent standard error which is the standard

200

deviation of the sampling distribution of a statistic, defined as the standard deviation divided by

201

the square root of the sample size – three independent assays unless otherwise specified.

202

RESULTS AND DISCUSSION

203

Molecular Weight Analysis. Lignin molecular weight is an important index to evaluate the

204

physical properties of lignin. The weight-average (Mw) and number-average (Mn) molecular

205

weight of CELF lignin samples are presented in Figure 1. The Mn and Mw of CEL isolated from

206

untreated poplar were 2850 and 11850 g/mol, respectively. As shown in Fig. 1, CELF lignin

207

fraction had significantly lower Mw and Mn compared to CEL regardless of pretreatments

208

conditions, suggesting dramatic depolymerization of lignin occurred during the CELF

209

pretreatments. CELF lignin samples also showed relatively lower polydispersity index (PDI)

210

compared to that of CEL (4.16). This indicated that CELF pretreatment led to relatively narrower

211

molecular weight distributions in soluble lignin. This is in agreement with other data reported

212

that organosolv lignin mainly contains small molecular weight fractions compared to untreated

213

native lignin.27-30 In order to understand the effect of pretreatment conditions (temperature, time,

214

THF/water ratio, acid catalyst, etc.) on molecular weights distribution of CELF lignin, certain 10

ACS Paragon Plus Environment

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

215

samples were further compared with each other. By comparing sample 7 with 14 and 15, it was

216

concluded that extending the pretreatment time from 15 to 30 min at 180 oC did not have any

217

significant impact on the lignin molecular weight, indicating that both depolymerization and

218

condensation reactions were equally favored during this period. However, extending the

219

pretreatment time from 30 to 60 min, it was found to have a significant impact on the lignin

220

molecular weight for CELF pretreatment performed at 170 oC, as GPC results indicated that 60

221

min pretreatment led to a ~27.1% decrease of lignin molecular weight compared to 30 min

222

pretreatment (sample 3 vs. 4). This suggested that cleavage reactions were favored over

223

repolymerization under this particular condition. Given the fact that CELF lignin from trial 7, 14,

224

and 15 had lower molecular weight than CELF lignin 3 and 4, it can be concluded that higher

225

pretreatment temperature appeared to have a more pivotal effect on lignin molecular weight than

226

longer pretreatment time. Comparison of sample 2 with 8 led to the conclusion that H2SO4

227

tended to generate lower molecular weight lignin than FeCl3. Increasing pretreatment

228

temperature from 170 oC to 180 oC (sample 3 vs. 15) caused a ~34.8% decrease of lignin

229

molecular weight while increasing the FeCl3 catalyst concentration from 0.05 to 0.15% (sample

230

8, 12, and 13) had a neglect effect on lignin molecular weight. Regarding the THF/water ratio,

231

relatively lower molecular weight lignin was obtained using 2:1, 3:1 and 7:1 THF/water ratio,

232

therefore although the impact is significant, no obvious trend can be obtained (sample 5 – 11) for

233

the impact of THF: water ratio. In conclusion, all lignin samples underwent significant

234

degradation during CELF process regardless of pretreatment conditions; pretreatment

235

temperature, acid catalyst, and THF/water ratio had important effect on lignin molecular weight, 11

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering

236

pretreatment time had moderate effect depending on the pretreatment temperature applied while

237

FeCl3 concentration had a negligible effect on CELF lignin molecular weight distribution. (a). 2500

(b). 3000 Mn

Mw

2000

Molecular Weight (g/mol)

Molecular Weight (g/mol)

Mn

1500 1000 500 0

Mw

2500 2000 1500 1000 500 0

CELF Lignin 3

CELF Lignin 4

CELF CELF CELF Lignin 7 Lignin 14 Lignin 15

(c). 2500 Mw

CELF Lignin 1

2000 1500 1000 500

CELF Lignin 2

CELF CELF CELF Lignin 8 Lignin 12 Lignin 13

(d). 14000 Molecular Weight (g/mol)

Mn Molecular Weight (g/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

12000 10000 8000 6000 4000 2000 0

0 CELF Lignin 5

CELF Lignin 6

CELF CELF CELF Lignin 9 Lignin 10 Lignin 11

Mn Mw Cellulolytic Enzyme Lignin

238 239

Figure 1. Weight average (Mw) and number average (Mn) molecular weights of CELF lignin and

240

CEL lignin. (a). Effect of pretreatment time and temperature. (b). Effect of catalyst and its

241

loading. (c). Effect of THF/water ratio. (d). Molecular weight of CEL. Sample information is

242

presented in Table 1.

243

HSQC NMR Analysis. In an effort to further investigate changes in the lignin structure

244

during CELF pretreatment, HSQC NMR analysis of selected CELF lignin samples was

245

performed. The representative HSQC spectra of CEL and CELF lignin (sample 5, 180 oC, 20 min, 12

ACS Paragon Plus Environment

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

246

0.1 M FeCl3, 1:1 THF/H2O ratio) are shown in Figure 2. The HSQC NMR spectra showed that

247

CELF lignin demonstrated very different structural features compared with poplar CEL. The

248

aromatic and aliphatic 13C/1H cross-peaks are assigned according to literature (Table S2).25 As

249

expected and consistent with literature, poplar CEL is primarily composed of syringyl (S) and

250

guaiacyl (G) units along with considerable amounts of p-hydroxyphenyl benzoate (PB) units.31

251

In aromatic regions, the cross peaks associated with S and G units were either reduced in

252

intensity or significantly shifted to condensed form in CELF lignin. In particular, the peaks

253

associated with condensed G2 and S2/6 were observed at δC/δH 112.4/6.72 ppm and 106.5/6.51

254

ppm, respectively, which is consistent with literature.32 In aliphatic regions, signals associated

255

with methoxyl group and β-aryl ether (β-O-4) inter-linkages appeared to be the most prominent

256

ones in CEL. The presence of phenylcoumaran (β-5) and resinol subunit (β-β) in CEL were also

257

confirmed by its C-H correlations. However, only trace amount of β-β interunit linkages were

258

detected and the remaining peaks for β-O-4 and β-5 were not apparent in CELF lignin as shown

259

in Fig. 2.

260

A semi-quantitative analysis using volume integration of contours in HSQC spectra was

261

further performed to calculate the monolignol compositions (such as S/G ratio) and relative

262

abundance of lignin interunit linkages (such as β-O-4, β-β, and β-5). For monolignol

263

compositions of S, G, and PB calculation, S2/6, oxidized and condensed S2/6, G2, condensed G2,

264

PB2/6 contours were used with G2 and condensed G2 integrals doubled. The Cα signals were used

265

for contour integration for the calculation of all the interunit linkages. Table 2 presents the

266

relative contents of lignin subunits as well as interunit linkages for poplar CEL and several 13

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

267

selected CELF lignin samples. The relative content of total S units (including

268

oxidized/condensed units) decreased while the content of total G units (including condensed

269

units) increased in CELF lignin samples causing a decrease of S/G ratio. The change in PB

270

content upon pretreatment was found to depend on the applied CELF pretreatment conditions.

271

Table 2 also confirms the dramatic cleavage of β-O-4 which is probably the most prominent

272

change in lignin structure following CELF.

273

Page 14 of 26

The observed scission of β-O-4, β-β, and β-5 interunit linkages are in part agreement with

274

those observed for conventional organosolv lignin using ethanol, methanol or acetone. It was

275

reported that organosolv lignin isolated from ethanol pretreated Miscanthus × giganteus (190 oC

276

and 60 min) had a ~55% decrease of β-O-4 compared with the untreated milled wood lignin

277

(MWL).30 Similarly, Hallac et al. reported that ethanol organosolv pretreated Buddleja davidii

278

(195 oC and 60 min) had a ~57% decrease in the content of β-O-4 compared to MWL.27 The acid

279

catalyzed cleavage of β-O-4 interunit linkages has been recognized as the major mechanism of

280

lignin depolymerization during acidic organosolv pretreatments.33 However, this type of

281

depolymerization is usually accompanied with re-polymerization or condensation reactions,

282

especially under acidic conditions.34 Thus, cleavage of lignin interunit linkages is generally

283

incomplete as there are competing solubilization and condensation pathways through the

284

carbocation intermediate. Herein, we reported a near complete removal of the common lignin

285

interunit linkages by an acid-catalyzed organosolv pretreatment using THF as the organic solvent

286

at relatively mild temperature (180 oC). The reason that THF can preferentially solvate lignin is

287

that the THF/water mixture can form a “theta” solvent system, in which the solvent-lignin and 14

ACS Paragon Plus Environment

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

288

lignin-lignin interactions are approximately equivalent in strength, leading to the lignin adopting

289

Gaussian random-coil conformations.21 Lignin in this type of coil conformation would not

290

self-aggregate, therefore it can be more easily broken down into smaller molecular fractions. The

291

reduction of β-5 could be attributed to the formation of stilbenes through the loss of γ-methylol

292

groups as formaldehyde.35

15

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

293 294

Figure 2. The representative HSQC NMR spectra of CEL (Top) and CELF lignin sample 5

295

(Bottom) including aromatic (Left) and aliphatic (Right) regions. S: syringyl; S’: oxidized

296

syringyl. 16

ACS Paragon Plus Environment

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

297

Table 2. Semi-quantitative analysis of lignin subunits and interunit linkages in poplar CEL and

298

CELF lignin samples. Lignin samples

CEL

CELF 1

CELF 2

CELF 5

CELF 8

Syringyl

60.9

56.8

44.3

54.2

52.4

Guaiacyl

39.1

43.2

55.7

45.8

47.6

p-hydroxybenzoate 14.3

13.6

17.5

17.7

14.0

S/G ratio

1.6

1.3

0.8

1.2

1.1

β-O-4

61.1

9.2

N/A

N/A

N/A

β-β

4.7

1.9

0.04

0.2

0.07

β-5

5.4

1.8

N/A

N/A

N/A

Lignin subunits (%)

Inter-linkages (%)

299 300

Content (%) expressed as a fraction of S+G. N/A: below detection limit. 31

P NMR Analysis. Lignin OH groups are important characteristics associated with lignin’s

301

negative role in enzymatic hydrolysis of lignocellulosics.36 Quantitatively 31P NMR analysis was

302

used to determine the amount of various types of hydroxyl groups in selected CELF lignin

303

samples, and results are compared to that of the CEL (Figure 3). The chemical shifts and

304

integration regions are summarized in Table S3. Aliphatic OH group is the dominant OH type in

305

poplar CEL which accounts for ~87% of the total free OH groups, and CELF pretreatment

306

resulted in a decrease of the content of aliphatic OH group and an increase of the content of total

17

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering

307

phenolic OH group. Among phenolic OH groups, C5 substituted OH was observed as the most

308

prominent OH type in all the CELF lignin samples. The amount of carboxylic acid group in

309

CELF lignin is also higher than that in CEL, which could be due to the hydrolysis of ester bonds

310

or oxidation of aliphatic OH groups during CELF pretreatment.29, 30 The dramatic increase of the

311

content of phenolic OH could be a result of β-O-4 interunit linkages scission as evidenced by the

312

HSQC analysis. The significant decrease of aliphatic hydroxyl groups could be attributed to the

313

loss of γ-methylol group as formaldehyde and OH groups on Cα,β or the whole side chain in

314

general to from stilbene structures ultimately.27 CELF lignin 1 had the highest amount of

315

aliphatic OH (2.57 mmol/mg) and lowest amount of total phenolic OH (3.06 mmol/mg), possibly

316

because of its low pretreatment severity. Increase in catalyst loading and THF/water ratio were

317

found to have a negligible effect on the lignin OH content. As free phenolic groups are essential

318

for antioxidant activity of lignin, CELF lignin might be suitable for application as antioxidants.37 7 CEL 6

CELF 1 CELF 2

5 OH content (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

CELF 5 CELF 8

4

3

2

1

0

319 18

ACS Paragon Plus Environment

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

320

Figure 3. Quantification of different hydroxyl group contents (mmol/g) of CEL and CELF lignin

321

samples determined by 31P NMR spectra.

322

Future CELF lignin Valorization Pathways. Lignin is the most abundant renewable

323

aromatic polymer on earth, offering great potential for high value-added applications in

324

polymeric materials and chemical development.38 Organosolv lignin is usually sulfur-free, rich in

325

functionality, and has limited carbohydrate contamination.39 Following the depolymerization of

326

lignin via CELF process to yield CELF lignin, a solvent-based extraction or solute-based

327

precipitation technique can be applied to further extract the low molecular weight components

328

from the high molecular weight components. The low molecular weight components of CELF

329

lignin could be a preferable candidate as a non-fuel product in polymeric materials. On the other

330

hand, a catalytic oxidative or reductive fragmentation could be applied on the cross-linked high

331

molecular weight fraction of CELF lignin to yield viable fuel, chemical, and pharmaceutical

332

precursors. In addition, CELF lignin could also be considered as a favorable lignin for structural

333

carbon fiber production because of its low content of reactive C-O bonds in interunit linkages

334

such as β-O-aryl ether.40

335

CONCLUSIONS

336

Sustainable pretreatment techniques need to be developed to extract lignin with high yield,

337

quality, and purity for an integrated biorefinery. Poplar was subjected to a Co-Solvent Enhanced

338

Lignocellulosic Fractionation process, and a new type of lignin stream named CELF lignin was

339

recovered and characterized for its physicochemical structural features. Results showed that

340

CELF lignin presented a decreased molecular weight, aryl ether interunit linkages, aliphatic 19

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

341

hydroxyl content and increased free phenolic and carboxylic acid hydroxyl groups compared

342

with the untreated native lignin. The scission of β-O-4 ether interunit linkages in CELF lignin is

343

the major mechanisms of lignin depolymerization during CELF pretreatment. CELF process

344

provides a potential value-added lignin stream to the biorefinery process, and it also serve as a

345

unique tool to study lignin chemistry in biomass pretreatments.

346

ASSOCIATED CONTENT

347

Supporting Information

348

The Supporting Information is available free of charge on the ACS Publications website at DOI:

349

Table S1. Composition analysis of CELF lignin samples.

350

Table S2. Assignments of lignin 13C-1H correlation signals observed in the HSQC spectra of

351

lignin samples.

352

Table S3. Typical chemical shifts and integration regions for lignin samples in 31P NMR spectra.

353

Table S4. Weight average (Mw) and number average (Mn) molecular weights of CELF lignin

354

and CEL lignin.

355

Table S5. Quantification of different hydroxyl group contents (mmol/g) of CEL and CELF

356

lignin samples determined by 31P NMR spectra

357

AUTHOR INFORMATION

358

Corresponding Author

359

*

360

Authors’ Contributions

Art J. Ragauskas. Fax: 865-974-7076; Tel: 865-974-2042; E-mail: [email protected].

20

ACS Paragon Plus Environment

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

361

The experiments were performed through contributions of all authors. All authors have approved

362

the final version of the manuscript.

363

Notes

364

The authors declare that they have no competing financial interests.

365

ACKNOWLEDGMENTS

366

The authors acknowledge funding support from Bioenergy Technologies Office (BETO) in the

367

Office of Energy Efficiency and Renewable Energy (EERE) under Award No. DE-EE0007006

368

with U.S. Department of Energy (DOE). The publisher, by accepting the article for publication,

369

acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable,

370

worldwide license to reproduce the published form of this manuscript, or allow others to do so,

371

for United States Government purposes. The Department of Energy will provide public access to

372

these results of federally sponsored research in accord with the DOE Public Access Plan

373

(https://www.energy.gov/downloads/doe-public-access-plan). The views and opinions of the

374

authors expressed herein do not necessarily state or reflect those of the United States

375

Government or any agency thereof. Neither the United States Government nor any agency

376

thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any

377

legal liability or responsibility for the accuracy, completeness, or usefulness of any information,

378

apparatus, product, or process disclosed, or represents that its use would not infringe privately

379

owned rights.

380

Abbreviations

381

CEL: Cellulolytic enzyme lignin 21

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

382

CELF: Co-Solvent Enhanced Lignocellulosic Fractionation (CELF)

383

FF: Furfural

384

G: Guaiacyl

385

GPC: Gel permeation chromatography

386

HMF: 5- hydroxymethylfurfural

387

LA: levulinic acid

388

MD: Molecular-dynamics

389

MWL: Milled wood lignin

390

Mw: Weight-average molecular weight

391

Mn: Number-average molecular weight

392

NMR: Nuclear magnetic resonance

393

NHND: endo-N-hydroxy-5-norbornene-2,3-dicarboximide

394

NREL: National Renewable Energy Laboratory

395

PB: p-hydroxyphenyl benzoate

396

PDI: Polydispersity index

397

S: Syringyl

398

SSF: Simultaneous saccharification and fermentation

399

TMDP: 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane

400

REFERENCES

401 402

1. Dutta, T.; Papa, G.; Wang, E.; Sun, J.; Isern, N. G.; Cort, J. R.; Simmons, B. A.; Singh, S., Characterization of Lignin Streams during Bionic Liquid-Based Pretreatment from Grass,

Page 22 of 26

22

ACS Paragon Plus Environment

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442

ACS Sustainable Chemistry & Engineering

Hardwood, and Softwood. ACS Sustainable Chemistry & Engineering 2018, 6, 3079-3090. DOI: 10.1021/acssuschemeng.7b02991 2. Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T., The Path Forward for Biofuels and Biomaterials. Science 2006, 311, 484-489. DOI: 10.1126/science.1114736 3. Zhang, M.; Chen, G.; Kumar, R.; Xu, B., Mapping out the structural changes of natural and pretreated plant cell wall surfaces by atomic force microscopy single molecular recognition imaging. Biotechnology for Biofuels 2013, 6, 147. DOI: 10.1186/1754-6834-6-147 4. Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y.; Holtzapple, M.; Ladisch, M., Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96, 673-686. DOI: 10.1016/j.biortech.2004.06.025 5. Pu, Y.; Hu, F.; Huang, F.; Davison, B. H.; Ragauskas, A. J., Assessing the molecular structure basis for biomass recalcitrance during dilute acid and hydrothermal pretreatments. Biotechnology for Biofuels 2013, 6, 1-13. DOI: 10.1186/1754-6834-6-15 6. Gillet, S.; Aguedo, M.; Petitjean, L.; Morais, A. R. C.; da Costa Lopes, A. M.; Lukasik, R. M.; Anastas, P. T., Lignin transformations for high value applications: towards targeted modifications using green chemistry. Green Chemistry 2017, 19, 4200-4233. DOI: 10.1039/C7GC01479A 7. Kumar, A. K.; Sharma, S., Recent updates on different methods of pretreatment of lignocellulosic feedstocks: a review. Bioresour Bioprocess 2017, 4, 7. DOI: 10.1186/s40643-017-0137-9 8. Tolbert, A.; Akinosho, H.; Khunsupat, R.; Naskar, A. K.; Ragauskas, A. J., Characterization and analysis of the molecular weight of lignin for biorefining studies. Biofuels, Bioproducts and Biorefining 2014, 8, 836-856. DOI: 10.1002/bbb.1500 9. Aghaziarati, M.; Soltanieh, M.; Kazemeini, M.; Khandan, N., Synthesis of tetrahydrofuran from maleic anhydride on Cu–ZnO–ZrO2/H-Y bifunctional catalysts. Catal. Commun. 2008, 9, 2195-2200. DOI: 10.1016/j.catcom.2008.04.022 10. Hunter, S. E.; Ehrenberger, C. E.; Savage, P. E., Kinetics and Mechanism of Tetrahydrofuran Synthesis via 1,4-Butanediol Dehydration in High-Temperature Water. The Journal of Organic Chemistry 2006, 71, 6229-6239. DOI: 10.1021/jo061017o 11. Sitthisa, S.; Resasco, D. E., Hydrodeoxygenation of Furfural Over Supported Metal Catalysts: A Comparative Study of Cu, Pd and Ni. Catal. Lett. 2011, 141, 784-791. DOI: 10.1007/s10562-011-0581-7 12. Fowles, J.; Boatman, R.; Bootman, J.; Lewis, C.; Morgott, D.; Rushton, E.; van Rooij, J.; Banton, M., A review of the toxicological and environmental hazards and risks of tetrahydrofuran. Crit. Rev. Toxicol. 2013, 43, 811-828. DOI: 10.3109/10408444.2013.836155 13. Cai, C. M.; Zhang, T.; Kumar, R.; Wyman, C. E., THF co-solvent enhances hydrocarbon fuel precursor yields from lignocellulosic biomass. Green Chemistry 2013, 15, 3140-3145. DOI: 10.1039/C3GC41214H 23

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482

Page 24 of 26

14. Nguyen, T. Y.; Cai, C. M.; Kumar, R.; Wyman, C. E., Co-solvent Pretreatment Reduces Costly Enzyme Requirements for High Sugar and Ethanol Yields from Lignocellulosic Biomass. ChemSusChem 2015, 8, 1716-1725. DOI: 10.1002/cssc.201403045 15. Nguyen, T. Y.; Cai, C. M.; Osman, O.; Kumar, R.; Wyman, C. E., CELF pretreatment of corn stover boosts ethanol titers and yields from high solids SSF with low enzyme loadings. Green Chemistry 2016, 18, 1581-1589. DOI: 10.1039/C5GC01977J 16. Cai, C. M.; Nagane, N.; Kumar, R.; Wyman, C. E., Coupling metal halides with a co-solvent to produce furfural and 5-HMF at high yields directly from lignocellulosic biomass as an integrated biofuels strategy. Green Chemistry 2014, 16, 3819-3829. DOI: 10.1039/C4GC00747F 17. Nguyen, T. Y.; Cai, C. M.; Kumar, R.; Wyman, C. E., Overcoming factors limiting high-solids fermentation of lignocellulosic biomass to ethanol. Proceedings of the National Academy of Sciences 2017, 114, 11673-11678. DOI: 10.1073/pnas.1704652114 18. Selig, M. J.; Viamajala, S.; Decker, S. R.; Tucker, M. P.; Himmel, M. E.; Vinzant, T. B., Deposition of Lignin Droplets Produced During Dilute Acid Pretreatment of Maize Stems Retards Enzymatic Hydrolysis of Cellulose. Biotechnol. Progr. 2007, 23, 1333-1339. DOI: 10.1021/bp0702018 19. Li, H.; Pu, Y.; Kumar, R.; Ragauskas, A. J.; Wyman, C. E., Investigation of lignin deposition on cellulose during hydrothermal pretreatment, its effect on cellulose hydrolysis, and underlying mechanisms. Biotechnol. Bioeng. 2014, 111, 485-492. DOI: 10.1002/bit.25108 20. Kumar, L.; Chandra, R.; Chung, P. A.; Saddler, J., The lignin present in steam pretreated softwood binds enzymes and limits cellulose accessibility. Bioresour. Technol. 2012, 103, 201-208. DOI: 10.1016/j.biortech.2011.09.091 21. Smith, M. D.; Mostofian, B.; Cheng, X.; Petridis, L.; Cai, C. M.; Wyman, C. E.; Smith, J. C., Cosolvent pretreatment in cellulosic biofuel production: effect of tetrahydrofuran-water on lignin structure and dynamics. Green Chemistry 2016, 18, 1268-1277. DOI: 10.1039/C5GC01952D 22. Smith, M. D.; Petridis, L.; Cheng, X.; Mostofian, B.; Smith, J. C., Enhanced sampling simulation analysis of the structure of lignin in the THF-water miscibility gap. PCCP 2016, 18, 6394-6398. DOI: 10.1039/C5CP07088K 23. Thomas, V. A.; Donohoe, B. S.; Li, M.; Pu, Y.; Ragauskas, A. J.; Kumar, R.; Nguyen, T. Y.; Cai, C. M.; Wyman, C. E., Adding tetrahydrofuran to dilute acid pretreatment provides new insights into substrate changes that greatly enhance biomass deconstruction by Clostridium thermocellum and fungal enzymes. Biotechnology for Biofuels 2017, 10, 252. DOI: 10.1186/s13068-017-0937-3 24. Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D., Determination of structural carbohydrates and lignin in biomass. National Renewable Energy Laboratory Analytical Procedure: Golden, CO, 2008. 25. Meng, X.; Pu, Y.; Yoo, C. G.; Li, M.; Bali, G.; Park, D.-Y.; Gjersing, E.; Davis, M. F.; Muchero, W.; Tuskan, G. A.; Tschaplinski, T. J.; Ragauskas, A. J., An In-Depth Understanding of Biomass Recalcitrance Using Natural Poplar Variants as the Feedstock. ChemSusChem 2017, 10, 139-150. DOI: 10.1002/cssc.201601303 24

ACS Paragon Plus Environment

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521

ACS Sustainable Chemistry & Engineering

26. Meng, X.; Evans, B. R.; Yoo, C. G.; Pu, Y.; Davison, B. H.; Ragauskas, A. J., Effect of in Vivo Deuteration on Structure of Switchgrass Lignin. ACS Sustainable Chemistry & Engineering 2017, 5, 8004-8010. DOI: 10.1021/acssuschemeng.7b01527 27. Hallac, B. B.; Pu, Y.; Ragauskas, A. J., Chemical Transformations of Buddleja davidii Lignin during Ethanol Organosolv Pretreatment. Energy & Fuels 2010, 24, 2723-2732. DOI: 10.1021/ef901556u 28. Sannigrahi, P.; Ragauskas, A. J.; Miller, S. J., Lignin Structural Modifications Resulting from Ethanol Organosolv Treatment of Loblolly Pine. Energy & Fuels 2010, 24, 683-689. DOI: 10.1021/ef900845t 29. Hu, G.; Cateto, C.; Pu, Y.; Samuel, R.; Ragauskas, A. J., Structural Characterization of Switchgrass Lignin after Ethanol Organosolv Pretreatment. Energy & Fuels 2012, 26, 740-745. DOI: 10.1021/ef201477p 30. El Hage, R.; Brosse, N.; Chrusciel, L.; Sanchez, C.; Sannigrahi, P.; Ragauskas, A., Characterization of milled wood lignin and ethanol organosolv lignin from miscanthus. Polym. Degrad. Stab. 2009, 94, 1632-1638. DOI: 10.1016/j.polymdegradstab.2009.07.007 31. Sannigrahi, P.; Ragauskas, A. J.; Tuskan, G. A., Poplar as a feedstock for biofuels: a review of compositional characteristics. Biofuels Bioprod Biorefin 2010, 4. DOI: 10.1002/bbb.206 32. Yoo, C. G.; Yang, Y.; Pu, Y.; Meng, X.; Muchero, W.; Yee, K. L.; Thompson, O. A.; Rodriguez, M.; Bali, G.; Engle, N. L.; Lindquist, E.; Singan, V.; Schmutz, J.; DiFazio, S. P.; Tschaplinski, T. J.; Tuskan, G. A.; Chen, J.-G.; Davison, B.; Ragauskas, A. J., Insights of biomass recalcitrance in natural Populus trichocarpa variants for biomass conversion. Green Chemistry 2017, 19, 5467-5478. DOI: 10.1039/C7GC02219K 33. Pan, X.; Xie, D.; Yu, R. W.; Lam, D.; Saddler, J. N., Pretreatment of Lodgepole Pine Killed by Mountain Pine Beetle Using the Ethanol Organosolv Process:  Fractionation and Process Optimization. Industrial & Engineering Chemistry Research 2007, 46, 2609-2617. DOI: 10.1021/ie061576l 34. Wang, C.; Li, H.; Li, M.; Bian, J.; Sun, R.; Sun, R., Revealing the structure and distribution changes of Eucalyptus lignin during the hydrothermal and alkaline pretreatments. Sci Rep 2017, 7, 593. DOI: 10.1038/s41598-017-00711-w 35. Li, S. M.; Lundquist, K., Acid reactions of lignin models of β-5 type. Holzforschung 1999, 53, 39-42. DOI: 10.1515/HF.1999.007 36. Pan, X., Role of functional groups in lignin inhibition of enzymatic hydrolysis of cellulose to glucose. J Biobased Mater Bio. 2008, 2, 25-32. DOI: 10.1166/jbmb.2008.005 37. Pan, X.; Gilkes, N.; Kadla, J.; Pye, K.; Saka, S.; Gregg, D.; Ehara, K.; Xie, D.; Lam, D.; Saddler, J., Bioconversion of hybrid poplar to ethanol and co-products using an organosolv fractionation process: Optimization of process yields. Biotechnol. Bioeng. 2006, 94, 851-861. DOI: 10.1002/bit.20905 38. Kumar, M. N. S.; Mohanty, A. K.; Erickson, L.; Misra, M., Lignin and its applications with polymers. J. Biobased Mater. Bioenergy 2009, 3, 1-24. DOI: 10.1166/jbmb.2009.1001

25

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

522 523 524 525 526 527

Page 26 of 26

39. Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M., Lignin valorization: improving lignin processing in the biorefinery. Science. 2014, 344. DOI: 10.1126/science.1246843 40. Foston, M.; Nunnery, G. A.; Meng, X.; Sun, Q.; Baker, F. S.; Ragauskas, A., NMR a critical tool to study the production of carbon fiber from lignin. Carbon 2013, 52, 65-73. DOI: 10.1016/j.carbon.2012.09.006

528 529 530 531

TABLE OF CONTENT

532 533 534 535 536

SYNOPSIS Co-Solvent Enhanced Lignocellulosic Fractionation pretreatment offers a promising approach for lignin isolation with unique characteristics.

26

ACS Paragon Plus Environment